What size rubber grommet will a 3/4″ PVC pipe fit (water tight) through

The schedule is the wall thickness of the pipe (essentially how much pressure it can withstand). Wikipedia has a Nominal Pipe Size Table (and the article goes into the description of the size and the why behind them).

From this article http://www.ehow.com/facts_5575151_difference-between-schedule-schedule-pipe.html:

For a relatively constant pipe outside diameter, the wall thickness varies from Schedule 10 (light) to Schedule 40 (standard weight) to Schedule 80 (extra strong) to Schedule 160 (double extra strong).

Schedule 40 pipe is typically used in construction. However, if a stronger pipe is needed, then Schedule 80 can be used. An example where Schedule 80 should be considered is if the pipe is exposed during construction. The stronger pipe is more resistant to incidental bumping by workers or equipment.

In my house the line that runs out to my irrigation system and the sewer pipe are all schedule 40 (the schedule is usually printed on the pipe).

Two part epoxy works well (maybe JB weld), just rough up the pvc a bit and clean it first. How much of a load are you talking about? If it is substantial, you may have to attach the PVC mechanically to the MDF.